Generalized stretch forming



sept. 27, 1966 B# J- ALECK 3,274,813

GENERALIZED STRETCH FORMING original Filed Mfh 28, 1962 s sheets-sheet 1 INVENTOR ENJ MIN J. CK BY TORNEYS.

B. J. ALECK sept. 27, 1966 GENERALIZED STRETCH FORMING Original Filed March 28, 1962 5 Sheets-Sheet 2 FIG. 4A.

s s e INVENTOR BENJAMIN J. 'ALECK ATTORNEYS.

sept. 27, 1966 B, J ALECK 3,274,813

GENERALIZED STRETCH FORMING Original Filed March 28, 1962 5 Sheets-Sheet 5 FIG. 6.

TRUE STRAIN (PERCENT DEEORMAT/ON) Eoge 03e loge IOB INVENTOR /BJENJAMIN J. ALE K BY .c W.

ATTORNEYS.

United States Patent Olice 3,274,813 Patented Sept. 27, 1966 3,274,813 GENERALIZED STRETCH FORMING Benjamin J. Aleck, Jackson Heights, N.Y., assignor to Arde-Portland, Inc., South Portland, Maine, a corporation of Maine Original application Mar. 28, 1962, Ser. No. 183,149, now Patent No. 3,197,851, dated Aug. 3, 1965. Divided and this application July 8, 1965, Ser. No. 470,367 2 Claims. (Cl. 72-62) This invention relates to metallurgical processing and particularly to a metallurgical processing for forming high tensile strength pressure vessels. More particularly, the present invention relates to a novel s-tep in metallurgical processing for form-ing high tensile strength pressure vessels vfrom metallic materials and particularly from metallic materials having vat least two distinct crystal structures, both of which can exist at room temperature. The present invention further rela-tes to apparatus for performing the novel step in the metallurgical processing.

This application is a division of my earlier filed copending application Serial No. 183,149' (now U.S. Patent No. 3,197,851), liled by me on March 28, 1962, which application is in turn a continuation-in-part of my earlier led now abandoned application Serial No. 834,439, led by me on Aug-ust 1-8, 1959, both of which applications are assigned to the assignee hereof.

There are many uses for high tensile strength pressure vessels and such uses are multiplying over the years. In a number of such uses, the Weight of the pressure vessel is of great importance. For instance, if the pressure vessel is to be carried by a man such as, for example, portable Oxy-acetylene tanks, it is desirable for the pressure vessels to be as light as possible and still meet the strength requirements.

One especially critical use for light-weight vessels is in the field of solid fuel rockets, where pressure vessels are employed to store the solid rocket 4fuel and to contain such fuel during ignition and combustion thereof. During such time, the vessel is subjected to extremely high pressures. In such applications it is obvious that failure of the vessel itself means failure of the rocket and, accordingly, considerable strength must be built into such pressure vessels. However, the weight of such pressure vessels is a vital factor in rocketry as the weight of the vessel itself will cut into the overall weight of the payload.` Accordingly, it is most desirab-le to create a high strength pressure vessel of very light weight. Naturally, if the material from which the material is made has a very high tensile strength, its overall strength and resistance to pressure will increase for a given weight of material. Thus light weight vessels can be made if extremely high tensile strength material is available.

There are many metallurgical processes avail-able today for strengthening steels and other metals suitable for pressure vessels and particularly for solid fuel rocket casings. For instance, cold rolled stainless steels are available and these materials have very high tensile strength after having been cold rolled. However, during the fabrication of vessels from such materials, it is generally necessary to weld the cold rolled stainless steel together to form the vessel. During .the Welding operation the metal in the zone of the welds, of co-urse, is subjected to high heat which heat generally destroys the gain in strength achieved by the cold rolling. Accordingly, the iinal vessel, though made out of material that generally speaking has high tensile strength, will have distinct low tensile strength zones in the vicinity of the welds which zones will limit the overall strength of the vessel.

When forming pressure vessels from materials that are dicult to weld, such as, for instance, titanium, it is often necessary to thicken the ends of lthe portions of the vessel blank which are to be butt welded to one another in order .to get additional strength in the weld. The need for additional strength is due not only to the difficulty of effecting a goo-d weld, but also due to the fact that the welding operation causes a loss in strength previously gained -by earlier employed metallurgical processes. However, the .technique of thickening the ends is extremely costly because in the present day art, the thickened ends of such vessels are formed by machining out material between the ends whereby to leave a flared or flanged end. This is an extremely expensive procedure due in part to the cost of machining and also due in part to the fact that the machining removes (and thus wastes) costly material, such as titanium.

The main object of the present invention is to provide a novel step in a metallurgical process for forming pressure vessels which step will produce an extremely high strength vessel for a given weight.

Still another object of the present invention is the provision of a metallurgical process for Working pressure vessels whereby to convert the metallic material forming the pressure vessel into a high tensile strength material.

Still ano-ther object of the present invention is the provision of a metallurgical process for forming welded high tensile strength pressure vessels.

Yet a further object of the present invention is the provision of a metallurgical process `for working pressure vessels made out of metallic materials having .two distinct phases or crystal structures both of which can exist at room temperature.

The above and other objects, and characteristics of the p-resent invention will be understood more fully from the following description taken in connection with the accompanying drawings.

ln -accordance with the present invention a pressure vessel blank is formed of metal, preferably by welding. Thereafter, the vessel blank is placed in a die of larger size than the blank itself. The vessel blank is then subjected to high internal pressure as by injecting or compressing a iiuid therewithin, the pressure being suicient to stretch .the metal from which the vessel blank is made. The stretching of the vessel blank will also cause a stretching of the welds and area surrounding the welds whereby to result in an increase in strength of the entire vessel blank including the Weld therein.

-In some instances, the pressure can be applied to stretch only the weld areas whereby to raise the strength of the weld areas.

The stretch forming technique may be used in combination with various metallurgical processes such as a heat treatment or precipitation hardening to further increase the streength gain in the pressure vessel. For instance, in materials exhibiting a martensitic transformation, if the stretch forming is performed at cer-tain temperatures, it can be employed to induce such martensitic transformation whereby to yield a very high tensile strength pressure vessel made in whole or part of martensitic material, including the weld areas.

ln the drawings:

FIG. l is a diagrammatic view of lan apparatus for processing pressure vessel blank at cryogenic temperatures;

FIG. 2 is a sectional View of a pressure vessel formed in accordance with the present invention disposed within a die;

FIG. 3 is a side elevational view of a pressure vessel which has been formed in the die shown in FIG. 2;

FIGS. 4A, 4B and 4C illustrate several steps in the method of forming an open ended pressure vessel having a threaded boss at the end thereof;

FIGS. 5A, 5B .and 5C are views of various steps in forming an open ended pressure vessel using 1a modified form of the novel method disclosed herein;

FIG. 6 is a true stress vs. true strain graph of typical ductile metals; and

FIG. 7 is an elevational view of a cylindrical welded vessel blank.

Referring first to FIG. 7, a conventional pressure vessel blank 100 is illustrated therein. This blank may be formed of a metal and, as shown, is fabricated by taking a sheet of said material to form a central cylinder 102 by means of a longitudinally extending weld 104. To enclose the vessel two vessel heads 106 and 108 are welded to the ends of the vessel blank 100 as by circumferential welds 110 and 112, respectively. The vessel heads 106 and 108 may be planar bu-t, preferably, are of convex configuration.

Assuming that the vessel 100 has been fabricated from a very high tensile strength material which was previously treated by some metal working process such as ausforming or cold rolling or the like, the overall strength of the vessel will still be far below that which would be theoretically possible from the tensile strength of the material prior to fabrication thereof. This sharp reduction in the strength of the vessel is due to the lfact that when the material is welded as at the welds 104, 110 and 112, the benefits of the metallurgical process used to raise the tensile strength of the material prior to `fabrication of the vessel blank 100 are generally all or at least partly l-ost because of the heat of welding. Thus, although most of the material will tbe extremely strong and of high tensile strength, the welds themselves and the zones immediately adjacent the welds will be sharply weakened whereby to provide zones of weakness which limit the -overall strength of the vessel.

In accordance with the present invention the vessel blank 100 is subjected to internal pressure of sufficient magnitude to exceed the elastic limit of at least the weld areas in the material whereby to cause a stretching of the welds and areas adjacent the welds and, preferably, of the overall material itself. Such stretching, as will be described in greater detail hereinafter, can improve the mechanical properties of the welds and of the material, or can effect a change in the crystallographic structure of the welds or material or can be used in combination with known metallurgical Iprocess to treat the entire Vessel blank after forming whereby to insure high tensile strength throughout the entire vessel including the welds. It will be obvious that upon the application of internal pressure, the .areas of minimum tensile strength, that is the areas surrounding and including the welds, will yield first and will continue to yield without any exceeding of the elastic limit of the stronger material uneffected by the welding operation until the strength of the welds has come up to the strength of the overall material. If pressure is continued to be applied after this point then the entire vessel blank including the now strengthened weld areas will continue to stretch to give additional strength to the overall vessel.

FIG. 6 shows a true stress vs. true strain curve for a `typical du-ctile work hardenable metal such as, for example, austenitic stainless steel. It will be noted that true stress is shown in pounds per square inch and true strain is shown in percent of deforma-tion (diametral stretching). As is Well known to those skilled in the art, true stress is equal to the tensile force on the material divided by the actual area of the material. True final length (l) 10g@ snaai length 1.)

original area AD) loge final area (Af) It will be seen that in the portion of the curve designated by the reference character the curve rises almost vertically and substantially linearly. This portion of the curve represents purely elastic deformation and the metal observes Hookes law. That is, if the stress is relieved lanywhere 'within this portion of the curve the metal will elastically return to substantially its initial shape. However, once the elastic limit has been exceeded at the point 122, the material tends to yield with small increase in true stress, this portion of the curve being observed to be substantially horizontal Iand running friom the point 122 to the point shown by the arrow 124. This approximately horizontal portion of the curve is designated by the reference character 126. In this area of :the curve there is considerable yielding of the material and Hookes law is not followed. Between the point dened by the arrow 124 and the point defined by the arrow 128 the curve shows a concave upward characteristic and it is in this area where work of a substantial amount commences to be put into Ithe material to sharply increase its tensile strength. This Iportion of the curve is designated tby the reference numeral 130. Beyond the concave upward portion 130, the true stress vs. true strain curve commences to follow a `sharp upwardly rising part which is substantially linear but slightly concave downward, which part of the curve is designated by the reference character 132. As stress increases this part of the curve becomes more .and more horizontal but never bends downwardly due to the fact that as soon as this tendency arises, the materiall breaks. Generally speaking, it is preferred to stress the material during stretching thereof into the portion of the curve part 132 between the arrows 128 and 134 wherein a substantial strength gain is achieved.

With continued reference to FIG. 6, it is presently necessary in order to achieve substantial strength gains to cause deformation beyond the arrow 124. That is, it is necessary to stretch the vessel blank sufficiently to move at least into the concave upward portion 130 of the true stress vs. true strain curve. Most preferably, it is desirable to stretch the vessel beyond the concave upward yportion of the curve and well into the concave downward portion 132 of the curve. For stainless steels and particularly for AISI No. 301 stainless steel, the point 124 is reached at about 5% unit deformation (i.e., e=5%), and this is the minimum deformation above which any substantial tensile strength gain can be achieved. For 301 stainless steel it is desired to stretch the material in the vessel blank somewhere between the l0 and 18 percent unit deformation (i.e., e=\l0% to 18%). However, some improvement in mechanical properties of the Vessel is achievable at deformations below point 124 (i.e., e 5%) and, for certain apeplications this may be desirable.

A number of metals display what is known as a martensitic transformation. The martensitic transformation is a rearrangement of the crystallographic structure without any change in the chemical composition of the crystal structure. It is diffusionless. Moreover, such transformations in materials in which they occur are spontaneous at certain temperatures. For instance, as the temperature of the material is dropped, a temperature point will be reached where a martensitic transformation will commence occurring spontaneously. This temperature is known as the MS temperature. The martensitic transformation will progress further as the temperature of the material continues to be dropped until at a certain temperature, Agenerally known as the Mf temperature, there will be maximum spontaneous martensitic transformation, that is as much martensite as can be for-med will be formed. It has been found that the martensitic transformation can be started above the Ms temperature if the material is deformed, that is, if mechanical work is put into the material. However, there is a maximum temperature above which no martensitic transformation will occur even if deformation takes place. This temperature is known as the Md temperature. Moreover, it has been -found that at temperatures below the Ms temperature, the martensitic transformation can be made to progress yfurther than it normally would spontaneously, provided the material is mechanically deformed at such a temperature.

In View of thi-s knowledge and finding, I have discovered that with pressure vessels made of materials which exhibit a martensitic transformation, it is desirable to stretch the Vessel blank at a temperature below the Md temperature, and preferably, close to the Ms temperature and most preferably at or slightly below the Ms temperature. When the stretching of such vessel blanks is performed at such temperatures, the vessel blank will gain in strength not only due to the stretching of the material, but also due to the crystallographic transformation of the material to the generally stronger martensitio phase. While it is preferred to stretch form materials exhibiting the martensitic transformation at about the Ms temperature at which temperature the material will be relatively ductile to thereby enable the processor to substantially deform the vessel blank While forming martensite, substantial strength gains can be achieved at temperatures below the Ms temperature at which there is a substantial amount of martensite already present and even at or below the Mf temperature at which all the martensite formable has been formed.

The following table lists the Ms temperatures for a number of materials:

Approximate Material: Ms temp. F.)

Titanium 1570 Aluminum-copper alloys (percent Al)- 11.5 740 13.0 430 15.0 150 Steel-AISI No.-

1034 740 1035 750 1050 610 1095 420 2340 520 3'190 300 4130 690 4330 630 4330-l-.17% V 610 4340 550 Iron-nickel alloys (percent Ni)- In addition to the materials listed in the chart, the austenitic stainless steels also exhibit a martensitic transformation and for these steels an equation exists to calculate the approxi-mate Ms temperature. This equation is as follows:

Utilizing the above formula, it has been found that with four stainless steels the following occurs: With a stainless steel having the following composition- Percent Mn 1.33 Si 1.49

Fe Balance the calculated MS temperature is 71 degrees F. The measured MS temperature is 156 F. The follow-ing are `other compositions of stainless steel with both measured and calculate-d MS temperature-s:

temperature calculated by the above presented formula of F. 'Ihe Ms temperatures as measured by standard techniques is 60 F. -For stainless steel having the following composition- Percent Cr 1 Ni 10.2

Mn 1.42 Si 0.46 C .023 N .042 Fe Balance the calculated Ms temperature is 297 'F. and the measured Ms temperature is about 320 F.

Another stainless steel has the following composition- Percent Cr A11.7 `Ni 14.8 Mn 1.25 ASi 0.33 C 0.052 N 0.035 -Fe Balance This material has a calculated Ms temperature ofl 479 F. (obviously impossible) and an actual measured Ms temperature of 452 F.

With all materials displaying a martensitic transformation, as already noted, the stretch forming preferably takes place at a temperature below the Md temperature and more preferably at a temperature at or below the Ms temperature, whereby to effect a martensitic transformation at least due to deformation and, within the preferred temperature range, partially by deformation and partially by the spontaneous transformation due to temperature alone. In any of these events, assuming that the vessel blank to be stretched has been welded, it will be seen that the Weld areas will also be stretched and the 7 martensitic ltransformation will take place therein as well as in the non-weld areas of the vessel whereby to greatly increase the overall strength of the pressure vessel, including the weld areas.

The step of stretch Iforming as described hereinbefore can also be employed in connection with other types of metallurgical treatments than those necessary to effect a martensitic transformation. For instance, with ferrous materials, conventional heat treatment techniques can be employed to effect a pearlitic transformation or a transformation to bainite, preferably a lower bainite, in conjunction with the stretch forming technique. For instance, if it is desired to stretch rform a pressure vessel blank and form bainite, then the pressure vessel blank will be heated above the equilibrium temperature of the vessel blank material and then quenched to a temperature in the isothermal ltransformation temperature range of the material to 'form bainite, with the lower portion of the range being desired, then maintaining the temperature of the vessel substantially constant while stretching it. While data for the determining of isothermal temperatures of various ferrous materials necessary to form bainite are readily available from standard time-temperature transformation curves (T-T-T curves), as an example, and not by way of limitation, .the following table of te-mperature ranges for typical materials is presented.

Isothermal transformation Material: temperature range, F. Nickel steel (19% Ni, 101 C.) 700 to 800 AISA 410 hardenable stainless steel 800 to 1300 AISA 4340 steel 6001 .to 1100 AISA 1080 steel 500 to 1100 During -the isothermal period of the process, the material will transform into bainite. Moreover, if the temperature tends to be towards the bottom of the range presented, the bainite will be what is commonly known as lower bainite, which is a very strong tough material. Naturally, with respect to transformation to bainite, this is only applicable to ferrous materials.

From the foregoin it will be seen that the stretch forming of pressure vessel blanks in combination with various -metallurgical processes will give rise to very strong high tensile strength press-ure vessels. Stretch forming is particularly useful with `respect to welded vessels in viewfof the fact that the welds of the vessel are treated along with the rest of the material of the vessel after formation of the vessel blank whereby to give high strength throughout the vessel, which is not ordinarily achievable by conventional techniques in which the vessel material is treated prior to formation into a pressure rvessel and the benefits of the .treatment are lost in the vicinity of welds when welding is performed. The gain Ifrom stretch for-ming may be due to an improvement in grain structure of the vessel blanks throughout, or the increase in yield point due to working, yor any of these in combination with gains coming from known metallurgical processes.

Various -techniques may be employed in stretch forming pressure vessel blanks into high tensile strength vessels, and these will now be described -in connection with the drawings and particularly in connection with FIGS. 1 through 51C thereof. Moreover, the method will be described in connection with a pressure vessel blank formed of stainless steel designated as AISI stainless steel Number 302 which has the following composition:

Austenitic iron Balance.

Certain of the stainless steels coming within the ranges of the AISI No. 302 stainless steel have an Ms temperature in the vicinity of 320 F. Accordingly, if it is desired during stretch forming to produce a martensitic transformation of such stainless steel, it will be necessary to stretch form the vessel blank preferably at the Ms temperature of 320 F. Of course, a certain amount of martensitic transformation can be achieved during stretch forming at higher temperatures up to the lMd temperature of the steel. However, it is preferred to work at or below the Ms temperature in order to maximize the amount of martensitic transformation.

To effect stretch forming at the M,s temperature of the 302 stainless steel, it is, of course, necessary to employ cooling apparatus in connection with the stretching apparatus, which apparatus is illustrated in FIG. 1 and is generally designated by the reference character 12. The vessel blanks which are made of 302 stainless steel are generally designated by the reference character 10.

The apparatus 12 includes a die 14 the internal surface of which is substantially identical to the final surface to be achieved for the finished pressure vessel to be formed from the vessel blank 10. Die 14 is disposed within a cold chamber 16 of an insulating chest 18 having a relatively thick thermal insulating wall 20 surrounding the chamber 16. Wall 20 is provided with a door or closure 21 to provide access to chamber 16. Door 21 may be hinged as at 23 and lhave a handle 25. Also disposed within chamber 16 are cooling coils or trays 22 here shown to be in the form of two at coils although a helical coil may be employed. The cooling trays 22 may be formed by taking a piece of tubing and bending it to follow a tortuous path. The cooling trays 22 are employed to cool the die 14 to the desired working temperature which is preferably about 320 F. in order to produce a martensitic transformation. It should be understood that cooling of the die, although advantageous, is not absolutely necessary to working the present invention as the important factor is the cooling of the vessel blank which may be achieved without a pre-cooling of the die.

Tor achieve the very low temperatures of the order of 320 F., a coolant or refrigerant such as liquid nitrogen is preferably employed. As shown herein the liquid nitrogen coolant is contained in a tank 24 having an outlet 26 which is connected to a conduit 28 for conveying the liquid nitrogen out of the tank 24. A valve 30 may be interposed in t-he conduit 28 to control the flow of the liquid nitrogen. As shown herein the liquid nitrogen is supplied to the cooling -trays 22 by a branch pipe 32 having a control valve 34 interposed therein. The conduit 28 is provided with a second branch pipe 36 which goes to the intake -of a pump 38 having an outlet 40 the ow through which is controlled by .a valve 42. The outlet pipe 40 extends through the insulating wall 20 of t-he cooling box 18 and into the chamber 16. In the chamber 16 the (pipe 40 passes through an opening 44 in the die 14 and is threadedly connected to the pressure vessel blank 10 at a threaded inlet 46 therein. The die 14 has an opening 48 at its other end to which is connected an outlet pipe 50 having -a valve S2 interposed therewithin to control the flow of fluids therethrough. If desired, the pipes 32, 36 andf40 may be provided with check valves 54 to limit the pressure therewithin. Furthermore, temperature measuring instruments 56 may be included to monitor the ternperature of various parts such as the die, the trays and the chamber. Moreover, if desired, a bleed valve 58 may be included in communication with the pipe 40 to relieve the pressure in the pressure vessel 10 at the end of the pressure step as will be described hereinafter. To prevent the refrigerant from picking up heat, pipes or conduits 28, 32, 36 and 40 may be thermally insulated as by insulation 59.

To form a pressure vessel in accordance with the present invention in apparatus 12, the valve 30 is rst closed and the pipes 32 and 40 are vented to remove liquid nitrogen from the latter two pipes. The door 21 in the insulating wall 20 of cold chest 18 is opened and the die 14 is also opened. The die 14 may be opened by providing it with a removable section. As shown in FIG. 1, die 14 has one of its hemispherical sections 60 secured to the central cylindrical section 62 thereof by threaded securing elements (not shown) which may be removed to permit the detachment of the hemispherical section 60 from the remainder of the die 14. Thereupon the pressure vessel blank may be disposed within the interior of the die 14. The method of forming the pressure vessel blank 10 will be described hereinafter in greater details. Suffice it to say at this time that the outer shape of pressure vessel blank 10 is of smaller size than the interior surface of the die 14. With the pressure vessel blank 10 disposed within the die the pipe 40 may be connected to the inlet 46 and the removable hemispherical section 60 may be secured to the remainder of the die 14 to thus close the die. Thereafter, the door 21 may be closed.

After the completion of the insertion of the blank 10 in the die 14, the valve 30 may be opened and the valve 34 may be opened .to permit liquid nitrogen in the tank 24 to flow under its oWn pressure into the cooling trays 22 to thus lower the temperature of the die 14. At the same time or shortly thereafter the pump 38 may be energized and the valve 42 may be opened lto permit liquid nitrogen under a pressure of the order of 2000 to 2500 p.s.i. to be fed through the pipe 40 into the interior of the pressure vessel blank 10. With the liquid nitrogen being supplied to the interior of the pressure vessel blank as well as to the cooling trays the temperature .of the die and the pressure vessel blank 10 will rapidly drop to approximately the temperature of the liquid nitrogen, that is 320 F. Moreover, with the liquid nitrogen being supplied to the interior of the pressure vessel blank 10 at high pressure the liquid nitrogen will force the pressure vessel blank t-o be stretched outwardly to conform the outer surface thereof to the inner surface of the die 14. This stretching may be of the order of 10% to 20% which is well beyond the elastic limit of the vessel blank material thereby resulting in a plastic deformation of the vessel blank. This degree of deformation at the working temperature of 320 F. Will cause a substantial amount of transformation to the martensitic phase to produce a substantial gain in strength over and above that produced by work hardening. Accordingly, when the pressure on the inside of the pressure vessel blank 10 is relieved the pressure vessel blank will not return to its original shape but will, save for a slight elastic shrinkage, substantially conform to the shape of the interior of the die 14.

Accordingly, after the application of the pressure on the interior of .the vessel by introducing liquid nitrogen therein under pressure, the pressure 4may be relieved by shutting oli the pump 38, closing the valve 42 and opening the relief valve 48 which will vent the liquid nitrogen in the tank to atmosphere. At the same time the valve 30 may be closed to discontinue supplying liquid nitrogen to the cooling trays 22. Thereafter, the door 21 in the wall may be opened, the hemispherical section 60 of die 14 may be removed from the remainder of the die and the pressure vessel, now stretched fully to size, may be removed from the remainder of the die and thereafter the method may be repeated.

It Will be understood that the pressure vessel, after being stretched to size, may be readily `removed from the die as after the pressure is relieved from the interior of the vessel, the vessel will shrink slightly within the elastic range and thus pull away from the wall of the die.

The purpose of the vent 50 should be explained. AS the die 14 is relatively well sealed, and as there is air between the outer surface of the pressure vessel blank and the inner surface of the die, when pressure is applied to the interior of the vessel blank 10 to cause a stretching thereof the air in the space between the blank and the die will be compressed. Unless this air is permitted to flow out between the space it Will cause some malshaping of the iinal pressure vessel. Accordingly, the vent 50 is provided to permit the air to flow from the space between the vessel and the die to the atmosphere through the valve 52.

The shape of the initial vessel blank 10 is of some importance in achieving a pressure vessel of the desired qualities. As shown in FIGS. 1 and 2 the vessel blank 10 has a uniform wall thickness and is made of two hemispherical end sections which are welded or otherwise connected to a cylindrical center section. While this particular shape will work satisfactorily for the formation of a closed ended high strength pressure vessel it can be demonstrated that hemispherical sections and cylindrical sections are stretched at different rates under uniform pressure. Specifically, the hemispherical sections cannot be stretched linearly as much as the cylindrical section for the same percent increase in area. Accordingly, in lieu of the shape of the blank 10 shown in fragmentary View in FIG. 1 a blank perhaps of the shape of a dog bone would be preferable in order to end up with a vessel having uniform strength throughout. Other shapes of vessel blanks may be calculated depending upon the desired shape of the end product.

While the method described hereinbefore is eminently suited to forming high tensile strength pressure vessels having closed ends and uniform wall thicknesses, it cannot be used readily as such to form an open ended vessel. If an attempt Were made to form an open ended vessel by making a vessel blank similar in configuration to but smaller in size than the open ended vessel desi-red,'when pressure is applied to the interior of the blank the open end would stretch and upon stretching would break any seal theretofore made to contain the pressure. With the seal broken, pressure would be lost and there would be relatively little stretching accomplished.

Accordingly, additional steps must be performed in order to manufacture an open ended high tensile strength vessel in accordance with the present invention. The simplest method of accomplishing this is to form a vessel blank having closed ends as has been described hereinbefore. The closed ended blank is then stretch formed in accordance With the aforedescribed method and the final vessel coming out of the die will be of a shape equivalent to the shape of two open ended vessels with their open ends joined together. Thereafter, the stretch formed vessel (which is really two connected open ended stretch formed vessels) may be cut along the line of connection of the two open ended vessels. After cutting, there will be two vessels of the desired shape. This is illustrated in FIG. 3 of the drawings, wherein a finished stretch formed lclosed ended vessel 10 is shown. The dotted line 64 in FIG. 3 is the line of juncture of two identical open ended vessels. Accordingly, when the stretch formed vessel 10 is cut along the line 64 two open ended vessels will be produced.

In lieu of simultaneously forming two open ended vessels as described in the preceding paragraph, a single open ended vessel can be so formed by fabricating a close ended vessel blank and stretching it as described above to the desired form except for the inclusion of the unwanted closed end. After stretching at sub-zero temperatures, the unwanted end can be removed las by cutting or otherwise to yield the desired open ended vessel.

Often times it is desirable to form a high tensile strength pressure vessel which has -a thickened wall portion. When such a vessel is desired the method described in connection with FIG, 1 must be modified, as with a portion of the wall of the vessel blank thicker than the -remainder of the Wall there may not be uniform stretching when pressure is applied. To overcome this difficulty supplementary means for forcing out or stretching the thickened Wall portion can be employed. Such a means may be, for instance, a multi-armed jack 74 as shown in FIGS. 4A and 4B. In lieu of the jack 74, having a multiplicity of angularly related distendable arms, a plurality of angularly yrelated jacks may be employed. Referring now to FIGS. 4A and 4B the vessel blank 10" is shown having two hemispherical end sections 66 and 68 of substantially uniform thickness and a central cylindrical section 70 having an inwardly directed ange 72. If such a vessel blank were placed in a die 14 and if pressure were .applied to the interior of the vessel blank to stretch it to conform to the inner surface of the die 14" the relatively thin walled sections of the vessel blank 10 would stretch out to conform to the die. But, the thickened wall portion represented by the flange 72 would resist such deformation. Hence, the end product would not be of the desired shape.

To overcome` this, while the vessel blank 10" is being formed the multi-armed jack 74 is disposed within the blank in operative engagement with the flange 72. After completion of the vessel blank 10" the blank may be disposed within the die 14 with the inlet for the pressure fluid lconnected to the pipe 40". The vessel blank may then be cooled either by cooling the die by such means as the cooling trays 22 or by introducing refrigerant into the interior of the vessel blank. After the rcooling of the vessel blank to the temperature as hereinbefore described, the jack 74 is operate-d to distend its arms and thus stretch the thick portion of the wall i.e. flange 72, of the vessel blank 10". Thereafter, pressure can be applied to the interior of the vessel blank to stretch the thin wall portions as hereinbefore described. In this way a pressure vessel having a portion of its wall of thicker dimension than the remainder can be formed.

If it is desired to form an open ended pressure vessel having a thick wall portion as shown in FIG. 4C then the finished vessel shown in FIG. 4B may, after removal from the die, be cut through its center section to yield two finished yopen ended .pressure vessels as shown in FIG. 4C. Often, the internal frange is utilized in open ended vessels to provide a threaded connection. Accordingly, in FIG. 4C, the flange 72 is shown provided with a thread 73.

It is possible to form a single open ended pressure vessel in accordance with the present invention. The open ended vessel may have a thickened wall portion or it may be provided with a uniform thickness throughout. The method of forming a single vessel will be substantially the same in either event. Such a method is illustrated in FIGS. 5A to 5C wherein a vessel blank having a flange 76 is shown. The ange 76 is adjacent the lopen end 77 of the vessel. In forming an open ended Ipressure vessel from an open ended pressure vessel blank 10", the vessel blank, after formation, is rst placed in `a die 14', the interior of which is formed to the shape desired for the final pressure vessel. The vessel blank is then cooled, preferably by cooling the die although dire-ct cooling may be employed. After cooling the open end 77 is stretched by mechanical means to cause it to assume the shape designed for it in the nal stretched vessel. The remainder of the vessel blank 10 at this point remains unstretched. However, with .the open end 77 now stretched to its final shape, a seal may be effected with the open end to subject the remainder of the vessel blank to stretching pressures, which seal will not be lost as the end 77 cannot stretch any further to break the seal. Accordingly, when pressure is applied, the remainder of the vessel blank will be stretched to cause it to conform to the shape of the die and thus form a finished open ended vessel.

Referring now particularly to FIGS. 5A, 5B and 5C la vessel blank 10" having an open end 77 is shown. As

shown herein the vessel blank 10" has a flange 76 adjacent the open end although, as indicated hereinbefore, this method is applicable to open ended blanks having uniform wall thicknesses as well. The vessel blank 10" as described is placed in die 14 the internal surface of which conforms to the nal shape of the vessel. The

4vessel blank 10 is then cooled. Preferably, although not necessarily, cooling is accomplished by cooling the die as by introducing refrigerant into cooling trays such as the cooling trays 22 as shown in FIG. l. When the vessel blank has been cooled to the desired sub-zero ternperature the open end 77 is stretched by mechanical means. Such a means may be a multi-armed jack like the jack 74 shown in FIGS. 4A and 4B. In lieu thereof a tapered mandril may be inserted into the open end 77 of the cooled vessel blank and a force applied to the shaft 80 connected to the mandril 78 to force the mandril inwardly of the blank and thus stretch out the open end 77. With the outer wall of the open end of the vessel blank stretched to engage the die a seal may be effected as by a plate 84 and an O-ring 86. Plate 84 is preferably provided with an inlet opening 88- to which the pipe 40 may be connected f-or conveying the pressure medium such as, for instance, liquid nitrogen from a pump to the interior of the vessel blank. After connection, the vessel blank may be stretched by introducing the pressure medium into the interior of the blank as hereinbefore described. After application of pressure to stretch the remainder of the vessel blank the pressure may be cut o and the stretched vessel may be removed from the die and it will be in the proper form and of extremely high tensile strength.

In connection with the modification described above with regard to FIGS. 5A to 5C, the mandril 78 may itself be employed to effect the seal of the open end. In such an event the mandril should be provided with an opening t-o permit the pressure medium, preferably the refrigerant under pressure, to be introduced into the interior of the ves-sel blank after stretching of the open end. Moreover, if a thickened wall portion is located at other than the open end, a suitable stretching means, such as multi-armed jack 74, may be employed to stretch such thickened wall portion separately from the stretching of the open end of the vessel and the thin wall portion thereof.

While it is presently preferred to apply the internal pressure to the vessel blank by means of the refrigerant under pressure as has been described in detail hereinbefore, the present method can be worked by utilizing other means -of applying pressure to the interior of the vessel blank.v For instance, explosions within a vessel blank may be employed to supply the necessary pressure for stretching the vessel blank to conform to the interior of the wall of the die. However, the advantages of utilizing the refrigerant itself are great. By utilizing the refrigerant itself as the vehicle or means for applying pressure to the interior of the vessel blank a very rapid cooling and an assurance of a maintenance of the vessel blank at very low temperatures is achieved. As a matter of fact, if the refrigerant serves as the pressure vehicle, it may be possible to eliminate die cooling means such as trays 22 and still cool the vessel blank to the desired temperature. Accordingly, it may be possible to reduce the cost of the equipment for working the present invention.

.Moreover, there is no danger of any corrosive condensations as might possibly occur if other fluids are injected into the interior of the vessel blank under pressure to effect the stretching. Furthermore, it will be apparent that s-ome fluids, such as, for instance, water, are inconvenient to use as they might freeze and thu-s prevent the application of the internal pressure. For these reasons the use of the refrigerant as the pressure means is the preferred embodiment.

In the example given above, both the weld area and the remainder of the vessel blank were stretched upon the application of .pressure to the interior thereof. However, it will be understood that a vessel blank can be fabricated from very highly tensile strength metal, the

vmethod of fabrication of the blank being as by welding.

However, upon the welding being employed to form the vessel blank, the benefits of the prior metallurgical processing for yielding the high strength material will be lost in the weld areas due to annealing in such areas, whereby to render a vessel blank made generally of high tensile strength material but with weakened areas in and around the welds. With such a vessel, all that is necessary to do is to stretch the weld areas t-o increase their strength with no need for stretching the remainder of the material making the vessel blank. Naturally, in View of the fact that the welds and the area surrounding them are substantially weaker than the remainder of the vessel blank, the welds will stretch first whereby to give the result desired.

One of the advantages of stretching only weld areas is realized when extremely large vessel blanks are to be formed. Generally speaking, the cost of building dies to house huge pressure vessels as might be employed as solid fuel rocket casing for an intercontinental ballistic missle will be so great as to render the process not readily useable. Accordingly, when fabricating such a vessel blank by using stretch forming techniques, what can be done is that a plurality of individual sections can be formed of very high tensile strength material such as cryogenically rolled. stainless steel. Each of the sections is essentially cylindrical. The sections can then be joined together as by welding whereby to weaken the material in the area of the welds but to render a very large vessel primarily made of extremely strong material. Thereafter, without the use of a die, pressure can be applied to the interior of the vessel so fabricated to stretch the weld areas and thereby gain strength in the weld areas. If it i-s desired to stretch the weld areas to effect a martensitic transformation of the material in the weld zones, then local temperature control in the area of the welds can Ibe effected to bring the weld areas close to the Ms temperature of the material prior to stretching whereby to induce a substantial degree of martensitic transformation upon stretching the weld areas. Accordingly, if the material from which the vessel was made is an AISI 302 stainless steel, it will be necessary to coolthe weld areas by liquid nitrogen or similar cryogenic material in order to bring the stainless steel in the weld areas down to its Ms Itemperature.

However, it will be seen that this type of stretching will not require a die and can be done out in the open with relatively simple equipment.

Furthermore, it is possible to have high tensile strength pressure vessels to which additional hardware must be secured after formation. Oft-times it is necessary to secure the hardware by welding or other process employing great heat, which process will tend to :anneal the vessel material in the area in connection with the additional hardware. Accordingly, in order to bring the vessel back up to strength after securing the hardware, it will -be necessary only to stretch form the vessel blank in theiarea where hardware has-been secured. rFhus, when working on very large vessels, or it may be necessary only to isolate certain portions of them for stretching whereby to cut down the amout of work to be done, the size of equipment, and so forth.

While I have herein shown and described the preferred form of this invention and have suggested several modifications therein, other changes and modifications may be made therein within the scope of the appended claims without departing from the lspirit and scope of this invention.

Having thus described Vmy invention, what I desire to secure and claim by Letters Patent is:

1. Apparatus for forming high tensile strength pres-y sure vessels, comprising a die adapted to receive a vessel blank and having at least a .portion of the interior of the size and shape of said vessel, said die having two openings therein, a container, refrigerant in said container, a pump having an inlet and an outlet, a first conduit connecting said container Iof refrigerant to said pump inlet, a second conduit extending from said pump outlet through one of -said openings in said die and `adapted to be connected in communication with the interior of said vessel blank for supplying said refrigerant under pressure to the interior thereof to cool and stretch said vessel blank to substantially conform to the interior of said die, and third conduit means for venting the space between said die and said vessel blank.

2. Apparatus for forming high tensile strength pressure vessels as defined in claim 1, further comprising means in spaced relation to the outside of said die for cooling said die, and a fourth conduit for supplying said last mentioned cooling means with refrigerant from said container.

References Cited by the Examiner UNITED STATES PATENTS 2,336,771 12/ 1943 Beam-an 72-342 2,344,779 3/ 1944 Kolderman et al. 72-62 2,861,530 11/1958 Macha 72-62 FOREIGN PATENTS 851,320 10/ 1960 Great Britain. 153,476 10/ 1963 Russia.

CHARLES W. LANHAM, Primary Examiner.

L. A. LARSON, Assistant Examiner, 

1. APPARATUS FOR FORMING HIGH TENSILE STRENGTH PRESSURE VESSELS, COMPRISING A DIE ADAPTED TO RECEIVE A VESSEL BLANK AND HAVING AT LEAST A PORTION OF THE INTERIOR OF THE SIZE AND SHAPED OF SAID VESSEL, SAID DIE HAVING TWO OPENINGS THEREIN, CONTAINER, REFRIGERANT IN SAID CONTAINER, A PUMP HAVING AN INLET AND AN OUTLET, A FIRST CONDUIT CONNECTING SAID CONTAINER OF REFRIGERANT TO SAID PUMP INLET, A SECOND CONDUIT EXTENDING FROM SAID PUMP OUTLET THROUGH ONE OF SAID OPENINGS IN SAID DIE AND ADAPTED TO BE CONNECTED IN COMMUNICATION WITH THE INTERIOR OF SAID VESSEL BLANK FOR SUPPLYING SAID REFRIGERANT UNDER PRESSURE TO THE INTERIOR THEREOF TO COOL AND STRETCH SAID VESSEL BLANK T O SUBSTANTIALLY CONFORM TO THE INTERIOR OF SAID DIE, AND THIRD CONDUIT MEANS FOR VENTING THE SPACE BETWEEN SAID DIE AND SAID VESSEL BLANK. 